There is great interest today in magnetic random access memory (MRAM) as a high performance, non-volatile memory using magnetic tunnel junction (MTJ) memory elements. The magnetic tunneling junction comprises two layers of ferromagnetic material separated by a thin insulating material. The direction of the magnetic moment of one of the layers is fixed, for example, by exchange bias with an antiferromagnetic material as described in U.S. Pat. No. 5,650,958, or by forming this layer from a ferromagnetic material with a large magnetic anisotropy, much higher than that of the second ferromagnetic layer, as described in U.S. Pat. Nos. 5,801,984 and 5,936,293.
The magnetic moment of this first layer responds very little to the magnetic fields which are applied during the operation of the memory device. By contrast, in the second ferromagnetic layer, the magnetic moment direction is allowed to move in response to magnetic fields which are applied during the operation of the memory device to set the direction of the magnetic moment of this layer. This applied field is referred to as the writing field or switching field.
It is advantageous if the magnetization of both the first and second ferromagnetic layers are largely homogeneous and aligned along one particular direction so that the magnetic moments form essentially a single magnetic domain. Usually the magnetic moment will not be homogeneous but, especially at the edges of the device where the demagnetization fields are largest, the magnetization may be directed away from the preferred direction.
Nevertheless, the direction of the magnetic moment in the second layer will largely either be parallel or anti-parallel to the first layer, thereby allowing the storage of data in the form of “ones” and “zeros”. The individual MTJs are generally formed by micro-lithographically patterning a continuous film comprised of the magnetic tunnel junction structure, and thereby defining an MTJ element with a particular shape and area. To create the MRAM the MTJ elements are incorporated within a network of “write” and “bit” lines for the purpose of reading and writing the MTJ devices as described in U.S. Pat. No. 5,640,343.
Electrical circuits for setting and interrogating the state of the individual MTJ elements may be formed by conventional CMOS processes. During the fabrication of the MRAM chip the MTJ elements will generally be fabricated by first depositing a continuous magnetic multi-layered film comprised of the magnetic tunnel junction structure on top of the array of write (or bit) lines which have been fabricated in a prior step. The MTJ film is then patterned using an etching technique to form the ultra-small tunneling junctions of a particular shape and size.
One of the major problems with MRAM using MJTs is scaling the MTJ elements to very small sizes. The switching fields of these elements become ever more sensitive to the detailed structure of the edges of these devices as the devices become smaller. Small variations in shapes and sizes of the MTJs and edge roughness of these devices cause large changes in the magnitude of field required for writing the individual device. Moreover, the field required to change the state of the MTJ memory element increases as the size of these elements decreases.
The magnetostatic fields emanating from the magnetic poles at the edges of the MTJ memory elements primarily determine the required magnitude of the writing or switching fields. The structure of the MTJ device can be made more complex to alleviate the role of the demagnetizing fields, for example, by forming one or both of the ferromagnetic layers comprising the MTJ from sandwiches of two or more ferromagnetic layers separated by thin non-ferromagnetic layers, as described in U.S. Pat. Nos. 5,841,692, 6,153,320, and 6,166,948.
In conventional MRAM cross-point architectures, the state of the storage elements is changed by passing currents through parallel arrays of “write-line” and “bit-line” wires near the MTJ memory elements. Typically, local fields at individual MTJ storage elements are created by passing electrical currents through two wires placed just above and just below the MTJ element.
An MTJ element is placed at the “cross-point” of each of the write and bit line wires. The write-line and bit-line wires are typically arranged to be orthogonal to one another with the MTJ element oriented with its easy magnetic axis oriented along one of the wires, usually referred to as the bit-line, although the MTJ device may also be oriented at some other angle with respect to the direction of the wires. A series of MTJ storage elements is arranged along each of the write and bit wires.
One selected element, at the cross-point of one of the bit-lines and one of the writelines, is written by passing currents simultaneously along these wires. Along these same wires there will be a number of half-selected elements which are subjected to either the write-line field or the bit-line field.
The maximum fields that can be generated by the currents passing along the write and bit-line wires is limited to about 100 Oe for reasonable current densities (or perhaps twice this amount if the wires are clad with a highly permeable soft ferromagnetic material such as permalloy). The maximum current is limited ultimately by electro-migration whereby atoms in the wires can be moved by current passing through the wire which can eventually lead to failure of the wire due typically to local necking of the wire which results in an increase in the local current density leading to a runaway process.
Thus, in actual devices the current density must be below this limit, but, even under these circumstances, the current density will be limited by other considerations, including requirements on power dissipation and the size of power transistors on the chip. Constraints on these currents thereby constrain the maximum field and so limit the smallest size of the MTJ elements.
The switching field can be decreased by reducing the net magnetic moment of the element, for example, by using magnetic material with smaller magnetization values or by using less magnetic material. Less magnetic material can be used by reducing the thickness of the magnetic layers in the MTJ device. However, these devices then become susceptible to thermal upsets due to the super-paramagnetic effect. Consequently, the MTJ elements must have sufficient magnetic anisotropy that they are stable against thermal fluctuations at the operating temperature of the MRAM device and, particularly, when these devices are half-selected during writing of elements in the MRAM cross-point architecture.
What is therefore needed is an MRAM architecture in which the switching of the memory elements is not determined by the shape and size of the memory elements, and in which much larger local magnetic fields can be generated, allowing smaller magnetic elements with sufficient magnetic stability against thermal fluctuations. The need for such a system has heretofore remained unsatisfied.